Cyclosiloxane mixtures have been isolated from the hydrolysis of mixtures of dimethyldichlorosilane and methyldichlorosilane under dilute solution conditions (Heimberger et al., Plaste Kautsch. 25(7), 386 (1978)). This process typically results in the formation of linear oligomers and branched and bicyclic oligomers, due to competitive Si--H bond hydrolysis. The process does not permit the isolation of any particular component in high yield.
Cyclosiloxane mixtures have also been prepared by the dilute solution equilibration of siloxanes. According to the equilibration mechanism, two or more types of siloxane units undergo disproportionation to the extent that placements of siloxane units is completely random within all cyclic and linear species. The result is an "equilibrated" fluid. If linear fluids are used as the source of the cyclics, low molecular linear oligomers are also produced. Such linear oligomers can be detrimental to many applications for these cyclic oligomers. The isolation of cyclic oligomers in a 93% yield by depropagation has been described for dimethylsiloxanes using sodium hydroxide as a catalyst at 240.degree. C. (Hunter et al., J. Amer. Chem. Soc. 68, 667 (1946)). Base catalysts may not be used with methylhydrogensiloxanes because of explosion hazards.
U.S. Pat. No. 2,860,152 and Canadian Patent 565,276 describe a method for producing cyclic diorganopolysiloxanes by heating a mixture of starting siloxanes in an inert solvent in an amount of at least 20% by weight based upon the weight of the siloxane, in the presence of an alkaline catalyst. More recently, U.S. Pat. No. 4,197,251 discloses the production of octamethylcyclotetrasiloxane by reacting dimethylsiloxane in the presence of an alkaline catalyst and inert solvent, while distilling cyclic dimethylsiloxanes.
U.S. Pat. No. 3,989,733 discloses production of cyclic polydiorganosiloxanes, particularly cyclic trisiloxanes, by thermal cracking and rectification over a hydroxide or carbonate of an alkali metal.
U.S. Pat. No. 2,439,856 and Canadian Patent 585,295 disclose the preparation of cyclic polymers of dimethyl silicone by hydrolyzing dimethyldiethoxysilicane in a 1:1 ratio with ethanol, in the presence of preferably an acid catalyst. The process results in cyclic polymers having up to thirteen silicon atoms.
U.S. Pat. No. 5,068,383 describes the catalyzed redistribution of monodispersed chloride and terminated polyorganosiloxane polymers. Acid clays may be utilized as the catalyst. Where cyclosiloxanes are produced, the reaction is run in the presence of stoichiometric or excess water in relation to the number of moles of starting chlorosiloxane to be redistributed.
U.S. Pat. No. 4,276,425 describes preparing cyclic dimethylpolysiloxanes by heating a mixture containing linear or branched organopolysiloxanes consisting of at least 50 mol% dimethylsiloxane units and aqueous H.sub.2 SO.sub.4. A very large amount of the aqueous catalyst is used, 0.4-1.5 liter per kilogram of organopolysiloxane reactant.
Cyclosiloxanes which contain methylhydrogen siloxy groups have been prepared by depropagating fluids at high temperature on supported inorganic catalysts such as acid clays at 298.degree. C. with 73% yield (U.S. Pat. No. 3,714,213); with acid clay catalyst and large quantities of water at 130.degree. C. with 61% yield (Japan Kokai and published Patent Application JP 52/69500 (1977)); and with sulfuric acid clays, sulfuric acid impregnated molecular sieves or acidic zeolites at temperatures in excess of 300.degree. C., 400.degree. C., and 500.degree. C., respectively under vacuum with a maximum of 86% yield (Crivello et al., Chemistry of Materials 1(4) 445 (1989); Crivello et al., U.S. Pat. No. 4,895,967). Large quantities of catalysts, often whose mass exceeds that of the siloxane reagents, are typically used. These processes are more efficient when the proportion of methylhydrogensiloxy units is high. The remaining reagents in the reaction mixture likely form a network residue due to radical cross-linking reactions which are known to occur readily at high temperatures, or due to Si--H hydrolysis and condensation reactions in the presence of water. Thus these methods do not readily permit continuous operation. The product that results generally has rings with 6 to 20 Si--O bonds with rings of 6 Si--O bonds predominating at high temperature. Six member ring siloxanes are strained and may not be used under many conditions where larger rings are stable. Except in the case of the acid zeolites, catalyst deactivation and contamination by sulfides is observed. Sulfide contamination is detrimental to subsequent hydrosilation reactions using transition metal catalysts. This is unfortunate, since transition metal catalyzed hydrosilation is one of the more important potential uses of cyclics mixtures.